Two of my favourite white costumes, that are also relatively rarely cosplayed (mainly because one of them is completely unknown to most people) are Dio Ereclea from Last Exile and Shiroi Kumo from Final Fantasy: Unlimited (sorry about the links, I don't have access to my own picture collections at work).

The princess from Murder Princess!!! I think it would be truly epic if someone cosplayed her, especially if after they were done with the dress, they would splatter it with blood, like from the first or second episode. You could even ask someone in metal shop (if you're in high school and they offer that class) to make you a crown that resembled hers!